1. Field of the Invention
The present invention relates to an injection molding machine having a check valve at a distal end portion of a screw, and more particularly to an injection molding machine that accurately detects backflow of resin as well as closing of the check valve when the screw is advanced.
2. Description of Related Art
In an injection molding machine provided with a check valve on a tip of a screw, in a case in which when metering is finished the check valve is open and in this state injection is carried out, a resin backflow occurs. As a result, any fluctuation in the timing of the closing of the check ring 3 causes a change in injection volume at each molding cycle, which affects the quality of the molded article.
FIG. 1 shows an example of the check valve. A check ring 3 capable of moving in the axial direction of a screw 1 is provided on a tip of the screw 1, at a portion of reduced diameter provided between a screw head 2 mounted on the tip of the screw 1 and body of the screw 1, and a check seat 4 that contacts and closely adheres to the check ring 3 to close a resin flow channel is provided on a screw 1 body side of the portion of reduced diameter. Resin pellets supplied from the rear of the screw 1 are supplied to the interior of a barrel 7 and are melted by shear heat generated by rotation of the screw 1 during metering and by heat from a heater provided on the outside of the barrel 7 in which the screw 1 is inserted. The melted resin causes the resin pressure behind the check ring 3 to increase, generating a force that pushes the check ring 3 forward. As the check ring 3 is pushed forward, resin at the rear passes through a gap between the check ring 3 and the portion of reduced diameter and flows in front of the check ring 3, increasing the pressure inside the barrel 7 in front of the screw head 2.
When the pressure in front of the check ring 3 exceeds a predetermined pressure (back pressure), the screw 1 is pushed back and the pressure in front of the check ring 3 is reduced. As the screw 1 rotates further the pressure behind the check ring 3 becomes higher than the pressure in front of the check ring 3, and the melted resin continues to flow to the front of the check ring 3. When the screw 1 retreats a predetermined amount, screw rotation is stopped, ending metering.
Next is the injection process, in which, as the screw 1 advances to fill a mold with the resin, the resin pressure building ahead of the screw head 2 increases, causing the check ring 3 to retreat and adhere closely to the check seat 4, closing the resin flow channel and preventing the melted resin from flowing backward (back-flowing) in the direction of retreat of the screw 1. The timing of the closing of the check valve fluctuates depending on the magnitude of the resin pressure in front of the check valve and the magnitude of the resin pressure behind the check valve when injection begins. From the start of injection to the closing of the check valve there occurs a backflow of resin from the front of the check valve toward the rear, which causes the timing of the closing of the check valve to fluctuate, which in turn causes the volume of injection at each molding cycle to fluctuate, affecting the quality of the molded article.
Accordingly, various methods have been proposed to detect the timing of the closing of the check valve, judging the quality of the molded article, and adjusting molding conditions.
For example, a method is known in which a pressure sensor is provided that detects the resin pressure inside the barrel at a position to the rear of the check valve, and detects the closing of the check valve based on changes in pressure detected by the pressure sensor (e.g., JP04-53720A and JP04-201225A). In addition, a method of advancing the screw as it rotates freely at the start of injection, allowing the backflow of resin to rotate the screw in reverse, and detecting a position at which this reverse rotation stops (e.g. JP2004-216808A) is also known.
Further, a method of displaying physical quantities relating to the molding operation as changing waveforms on a display, monitoring the injection molding operation, and detecting molding irregularities is also well known. Thus, for example, a method of detecting injection pressure waveforms in the injection process and superimposing multiple such pressure waveforms in an overlapping display to facilitate detection of irregularities is also known. In such overlapping display, reference points are set and the points at which the detected pressure waveforms pass the reference points are allowed to coincide with each other to provide the overlapping display and facilitate comparison of the waveforms (JP2006-247874A).
The amount of backflow of resin during injection affects the volume (weight) of resin that fills the interior of the mold, and therefore it is preferable that the backflow amount be constant at each of the molding cycles. Accordingly, the conventional art described above uses the resin backflow amount as a benchmark to detect the timing of the closing of the check valve and carry out determination of the backflow amount, judgment of the quality of the molded article, and adjustment of the molding conditions.
In addition, the molding states of the molding cycles can be detected and the quality of the molded article can be judged by detecting physical quantities associated with the molding operation, such as injection pressure in the injection process, and providing an overlapping display of waveforms of changes in these physical quantities as described in JP2006-247874A. However, detecting the injection pressure at each cycle and providing an overlapping display of the detected injection pressure waveforms in order to determine if the resin backflow amount and the timing of the closing of the check valve themselves are reliably constant so as to be able to detect irregularities is difficult.
The reason is this: Although in a typical injection molding machine a pressure sensor for detecting the injection pressure is provided to the rear of the screw (that is, to the right of the screw in FIG. 1), what is detected by this sensor is pressure exerted on the screw in the axial direction of the screw, and therefore it is difficult to detect a force in the axial direction exerted by the back-flowing resin on the flights of the screw by the resin pressure in front of the screw head. Consequently, it is difficult to detect accurately the resin backflow state from the detected injection pressure.